Continuous sulphate pulping method with countercurrent heating and concurrent cooking zones and apparatus therefor



July 6, 1965 D E. BENJAMIN CONTINUOUS SULPHATE PULPTNG METHOD WITH COUNTERCURRENT HEATING AND CONCURRENT COOKING ZONES AND APPARATUS THEREFOR Filed OCT. 29, 1962 28 66 1:". 7 .rman, L2 1:9

LIQUOR l f mmummnn 3 V PULP 42 WHITE 6| 0 44 LIQUOR g 10 k J 2 x2 g Ty):- ll'HHHll- Hllllli 1 {LII SPENT 1; CHIPS LIQUOR I In 73 z. 7| V I K? 113 j -J-w 12 f I United States Patent 3,193,444 CONTENUQUS SULPHATE PULPING METHGD WETH (IQUNTERQURRENT HEATENG AND CON- tCURRENT CGUKKNG ZGNES AND APPARATUS THEREFOR Dale E. Benjamin, Lewiston, Idaho, assignor to Potlatc'n Forest, lino, Lewiston, Idaho, a corporation of Delaware Filed Oct. 29, 1962, Ser. No. 233,534 5 (Ilaims. (Cl. 1162-49) This invention relates to continuous pulping of fibrous material and more particularly to novel methods and apparatus especially useful for conducting the sulphate rocess of pulping fibrous materials.

In the sulphate process for pulping fibrous materials such as wood chips, it has been the practice to'charge the fibrous material into a pressurized vessel along with sufficent White liquor to carry out the pulping reaction, and,

additionally, with suificient black or spent liquor, to dilute the chemicals and submerge the chips. In continuous .digesters the chips and liquors are usually fed continuously into one end of the system, and cooked pulp and spent liquor are discharged from the other end, although countercurrent flow has sometimes been used as well.

One problem which has always been a problem in the sulphate process is that the spent liquor at the end of the process still contains a high proportion of reactive chemical, which must be separated from the pulp and recirculated through the recovery plant and finally returned to the digester with the White liquor. This portion of the reactive chemical serves no function and thus constitutes a useless load on the recovery and liquor-making plants, increasing their required size and cost.

Another problem With conventional methods is that the spent liquor discharged with the pulp contains a large proportion of the heat which has been put into the process in order to reach the required reaction temperature, and this heat is difiicult to recover in an efficient manner. In batch digestion systems this heat is either wasted or is recovered from steam given off in the blow tank by using it to heat water for use elsewhere in the plant. This latter system requires large heat exchangers and accumulator tanks, greatly increasing the cost of installation. In continuous digesters, various designs have been used to recover heat from spent liquor so that it could be used at the beginning of the process, but these also involve heat exchange equipment which is both expensive to install and troublesome to maintain.

It is an object of the present invention to reduce the quantity of steam required to heat the fibrous material being processed in a continuous pulp digester.

A further object of the invention is to conduct the pulping process using much less treating chemical than has previously been possible.

A still further object of the invention is to conduct an improved sulphate pulping process in a single digester vessel.

It is also an object of the invention to utilize the countercurrent heating zone of a continuous digester for recovery of heat from spent liquor so as to eliminate the need for conventional heat exchangers.

According to the present invention, I have provided a novel method and apparatus for recirculating a portion of the spent liquor from the digested fibrous material back through the raw material at the beginning of the process in such a way that both the heat and the residual chemical are recovered in large degree from the spent liquor. Furthermore, the invention provides novel apparatus whereby this process is carried out in a single continuous digester vessel without the need for expensive auxiliary heat exchangers or other equipment.

Specifically, this invention provides a method whereby wood chips are continuously digested while maintaining an extended column of these chips in a single pressurized vessel, while treating the Wood chips in successive stages Within said column. A heating stage is provided adjacent to the inieed end of the column, a cooking stage is provided spaced from the infeed end, and a washing stage is preferably provided adjacent the discharge end of the column. Chemical treating liquor is introduced between the heating and cooking stages, and a portion of this liquor is caused to flow through the heating stage in the opposite direction to the movement of the wood chips. Another portion of the liquor is caused to flow through the cooking stage in the same direction as the cooked chips. The spent liquor mixed with hot wash liquor when the latter is used, is drawn oil at the end of the cooking stage and is reinjected with the chemical treating liquor between the heating and cooking stages for co-current flow through the cooking stage and countercurrent flow through the heating stage. The portion of liquor which flows through the heating stage countercurrent to the wood chip movement is drawn off at the infeed end of the chip column.

A major feature of the invention is that the liquid drawn oil from the infeed end of the chip column has given up heat and residual chemicals to the wood chips moving in the opposite direction so that heat and reactive chemical is recovered within the column of chips immersed in liquid within the single pressurized treating vessel and does not leave the process.

For the purpose of fully explaining preferred embodiments of the invention, reference is now made to the following specification, taken together with the accompanying diagrammatic drawing in side elevation of a preferred apparatus according to the invention.

Referring to the drawing, the pressurized vessel 22 therein shown is generally similar to that shown in U.S. Patent No. 2,878,116, issued March 17, 1959, but further includes according to the present invention means for establishing a heating zone adjacent the input end of the vessel, a cooking zone spaced from the input end of the vessel by said heating zone, and, preferably, a washing zone adjacent the output end of the vessel, together with means for removing liquid from the end of said cooking zone remote from said heating and cooking zones and for reintroducing at least a portion of said removed liquid between said heating and cooking zones, and means for removing liquid from the input end of said vessel, causing liquid flow throughout said heating zone in a direction toward the input end of the vessel countercurrent to the direction of chip movement and throughout said cooking zone in a direction away from the input end of said vessel co-current to the direction of chip movement. In general, the apparatus includes a generally cylindrical upright elongated pressure vessel 12 of gen erally uniform cross-sectional area at least not decreasing in its upward direction having a bottom inlet opening 18 connected to pump 16 and a top discharge mechanism connected to a suitable pulp storage tank (not shown). A chip input tank 14- is also provided, as well as a fluid outlet 15 from vessel 12 for removing liquor therefrom for feeding to chip tank 14 and pump 16. The advancing means includes a foraminous helical plate means 20 mounted for rotational and reciprocatory movement through a limited distance relative to the length of the vessel withinthe reaction vessel 12 near the lower end thereof by means of a shatt 3% extending vertically through the bottom of said vessel generally axially thereof. More specifically, said foraminous helical plate means comprises a single turn helical plate 20 having therein a plurality of perforations sufiicient in number to allow substantially free liquor passage. Furthermore, the step of such helical surface is closed by a fiat plate extending vertically between its upper and lower edges, and the peripheral edge thereof is positioned closely adjacent the innerwalls of the reaction vessel 12 so that passage of chips into the bottom portion of said reaction vessel 12 may be prevented.

For feeding the pressurized mixture of chips and liquid to the upper surface of said plate, a tubular member 24 having its inner wall concentric with and spaced from shaft 30 is mounted on the lower surface of said helical plate extending downwardly therefrom, thus providing a central aperture between the upper and lower radial edges of said helical plate. The lower end of shaft 30 extends into a hydraulic cylinder (not shown) mounted beneath the lower end of reaction vessel 12 and forms the piston thereof, said cylinder being connected to a suitable source of fluid pressure to raise said shaft with the helical plate to reciprocate said plate, the force of the vessel pressure against the area of the upper end of said shaft being great enough to rapidly lower the plate when the pressure in the cylinder is exhausted, the pressure vessel 12 in such circumstances acting as an accumulator to provide the necessary force at a rapid rate.

The top discharge mechanism comprises a scraper 26 located near the top of the vessel adapted to remove digested wood chips'therefrom and so define the upper end of the mass of chips Within vessel 12, as well as to define the'upper end of the chip drainage zone. Scraper 26 moves the chips through housing 28, and discharges the chips to a vertical chamber 31. In the bottom zone of this chamber an upward facing, bladed agitator 40 is rotatably mounted and operatively connected with a driving means :2. A chamber bottom surface 44generally conforms with the path of the lower edge of the rotatable blades, to define the lower extremity of the bottom zone. At least one orifice 46 is positioned in the bottom zone of the chamber in the bottom surface 44 below the path of the bladed agitator 40 so that rotation of the bladed agitator causes each blade to wipe by the inlet to the and to define the top of the pulping or cooking zone and.

thebottom of the submerged washing zone thereabove, if such be used. A liquid level sensing device may be suitably located to sense and establish a liquid level above the strainer 60 and below the discharge opening 28. It is also essential that inlet opening means, preferably in the form of a series of inlet openings 62 be provided in said vessel below strainer 60 and above chip lifter plate 20, with recirculating means including pipe 64, valve 66 and pump 68 connecting upper strainerv 60 and inlet openings 62. An inlet including valve 61 for a chemical treating agent and an inlet including a valve 63 for steam is also provided connected to inlet openings 62. Outlet 15 and plate 20 may be utilized as means for running the spent liquor from the inlet end of vessel 12 through pipe 71 and valve 73, but it is preferred to utilize a separate strainer 70 having a valve 72 therefor, said strainer 70 being spaced a substantial distance below inlet openings 62. Preferably, too, as shown, a pipe 74 having a valve 76 may be utilized to return a portion of the spent liquor from strainer 60 to chip tank 14. In addition, if desired, a series of wash liquid inlet openings 76 peripherally spaced in a horizontalrplane around the vessel generally midway between the strainer 60 and discharge opening .28 are provided, preferably adjacent the 4 top of the submerged washing zone and bottom of the drainage zone defined by the level sensing device.

In operation, wood chips are introduced into tank 14, with liquid being added thereto either from outlet 15 or pipe 74, the wood chip liquid mixture being pressurized and moved by pump 16 through inlet 18 and onto the upper surface of plate 20, establishing and maintaining .a vertical compacted straight column of wood chips in a pressurizedfluid medium while advancing wood chips throughout the length of the column from its input end at plate 20 to its output end at scraper 26. The entire length of vessel 12 being unobstructed, plate 20 acts to advance the wood chips by exerting Wood chip advancing force only on the lower input end of the column of chips, wood chips being introduced to the input end of the column by pumping 16 and plate 20; The cooked wood chips are discharged from the end of the column by scraper 26, and thereafter are passed as wood pulp to further stages through valve 48.

According to the methods of the invention, the wood chips, while maintained in a column as described, are treated in at least two successive zones in .said column, a heating zone adjacent the input end of the column, extending downwardly from inlets 62, and a cooking zone spaced from the input end of the column by the heating zone, and extending between inlets 62 and upper strainer 60, above the heating zone. Spent liquor is removed from strainer 60, and at least a major portion of it is recirculated by pump 68 from the end of the cooking zone remote from the heating zone and reintroduced at inlets 62 between the heating and cooking zones. The major portion of the spent liquor is removed from the system at the input end of the column, prefer- Y ably through strainer 70 and valve 72 for recovery, al-

though it might also be removed at outlet 15, through pipe 71 and valve 73.

A chemical treating agent, such as an alkali or a mixture thereof including, in a sulfate cooking process, white liquor consisting of a solution of sodium hydroxide, sodium carbonate, and sodium sulfide (NaOH, Na CO and Na s), is added through valve '61 together with a heating fluid, such as steam through valve 63, to give a final concentration vat inlets 62 between about 25 to 65 grams per liter of active alkali, expressed as Na O. The

temperature of the fluid introduced through inlets 62 is at a minimum of 315 degrees F. The spent liquor is discharged at the inlet end of the chip column at a temperature of no more than 212. degrees F., preferably less than degrees F., and at a concentration of active alkali of no more than 4.0 to 6.0 grams per liter. The low temperature of the spent liquor discharge and the low concentration of the alkali thereat are characteristic of the methods of the invention, and are a substantial improvement over heretofore known methods.

A third zone may be added in accordance with the methods of the invention by superimposing a diffusion washing zone at the outlet end of the chip column, Wash liquor being added through inlets 76 and passing toward the inlet end of the column countercurrent to the direction of chip movement before being removed with the spent liquor at strainer 60.

If desired, a portion of the spent liquor, with the added wash liquor if such be present, may be added through valve 76 and pipe 74, to the chips in tank 14 prior to their introduction into vessel 12 at the inlet end of the chip column. Since this spent liquor is at a higher temperature and chemical concentration than that taken off through valve 72, it gives up some heat and chemicals to the incoming wood chips, further increasing the efficiency of the process.

Various modifications of the methods and apparatus of the invention will be apparent to those skilled in the art.

I claim:

1. A methodfor the continuous cooking of wood chips comprising continuously maintaining a vertical column of Wood chips in a fluid medium while advancing wood chips throughout the length of said column from the input to the output end thereof introducing wood chips to the input end of said column and discharging cooked chips from the output end of said column meanwhile treating said wood chips in two successive zones in said column,

a heating zone adjacent the input end of said column and a cooking zone spaced from the input end of said column by said heating zone,

removing liquid from the end of said cooking zone remote from said heating and cooking zones and introducing a heated chemical agent comprising a solution of sodium hydroxide, sodium carbonate, and sodium sulfide and reintroducing at least a portion of said removed liquid at a location within the column between said heating and cooking zones and removing liquid from the input end of said column thereby causing liquid flow throughout said heating zone in a direction toward the input end of said column and throughout said cooking zone in a direction away from the input end of said column.

2. A method for the continuous cooking of wood chips comprising continuously maintaining an extended vertical compacted straight column of wood chips in a fluid medium while advancing wood chips throughout the length of said column from the lower input end thereof to the upper output end thereof exerting wood chip advancing force directly only on the lower input end of said column while introducing wood chips in a fluid medium to the lower input end of said column and discharging cooked chips from the upper output end of said column meanwhile treating said wood chips in two successive zones in said column,

a heating zone adjacent the input end of said column and a cooking zone spaced from the input end of said column by said heating zone,

removing liquid from the end of said cooking zone remote from said heating and cooking zones and reintroducing at least a portion of said removed liquid at a location within the column between said heating and cooking zones and introducing a chemical treating agent comprising a solution of sodium hydroxide, sodium carbonate and sodium sulfide to the column between said heating and cooking zones, heating the chemical treating agent and said portion prior to the introduction thereof into said column between said heating and cooking zones removing liquid from the input end of said column causing liquid flow throughout said heating zone in a direction toward the input end of said column and throughout said cooking zone in a direction away from the input end of said column.

3. A method of the continuous cooking and washing of wood chips comprising continuously maintaining an extended straight vertical column of wood chips in a fluid medium while advancing wood chips throughout the length of said column from the input end thereof to the output end thereof exerting wood chip advancing force directly only on the input end of said column while introducing wood chips to the input end of said column and discharging cooked chips from the output end of said col-umn meanwhile treating said wood chips in three successive zones in said column,

a heating zone adjacent the input end of said column a cooking zone spaced from the input end of said column by said heating Zone, and

a washing zone adjacent the output end of said column removing liquid from the end of said cooking zone remote from said heating and cooking zones and heating and reintroducing at least a portion of said removed liquid into said column between said heating and cooking zones and introducing a heated chemi cal treating agent comprising a solution of sodium hydroxide, sodium carbonate and sodium sulfide into said column between said heating and cooking zones introducing washing liquid to said washing zone at the output end of said column removing liquid from the input end of said column causing liquid flow throughout said heating zone in a direction toward the input end of said column and throughout said cooking zone in a direction away from the input end of said column.

4. Apparatus for the continuous cooking of wood chips comprising:

wood chip lifting means mounted adjacent the entrance end of said vessel to exactly lifting force directly only on wood chips located at the entrance end of said Vessel progressively to compact said wood chips and to advance the mass of said wood chips progressing upwardly throughout said zones while permitting free movement of liquid through said mass,

Wood chip discharge means adjacent the upper end of said vessel eliective to maintain a predetermined level of the top of said mass of wood chips below the top of said vessel,

lower strainer means adjacent the bottom of said vessel upper strainer means adjacent the top of said vessel for removal of liquid from said vessel inlet means for introducing a heated chemical treating agent into said vessel, at a location spaced between said upper and lower strainer means,

means for recirculating liquid from said upper strainer means into said vessel at said location within said vessel spaced between said upper and lower strainer means said inlet means and strainer means causing liquid flow in a downward direction from the injection point of said liquid to said lower strainer means and in an upward direction from the injection point of said liquid to said upper strainer means.

5. Apparatus as claimed in claim 4, further including washing liquid inlet means spaced above said upper strainer means said washing liquid inlet means and said inletmeans causing countercurrent liquid flow in a downward direction between said washing liquid inlet means and said upper strainer means.

References Cited by the Examiner UNITED STATES PATENTS 8/48 Sweden.

DONALL H. SYLVESTER, Primary Examiner.

MORRIS O. WOLK, Examiner. 

1. A METHOD FOR THE CONTINUOUS COOKING OF WOOD CHIPS COMPRISING CONTINUOUSLY MAINTAINING A VERTICAL COLUMN OF WOOD CHIPS IN A FLUID MEDIUM WHILE ADVANCING WOOD CHIPS THROUGHOUT THE LENGTH OF SAID COLUMN FROM THE INPUT TO THE OUTPUT END THEREOF INTRODUCING WOOD CHIPS TO THE INPUT END OF SAID COLUMN AND DISCHARGING COOKED CHIPS FROM THE OUTPUT END OF SAID COLUMN MEANWHILE TREATING SAID WOOD CHIPS IN TWO SUCCESSIVE ZONES IN SAID COLUMN, A HEATING ZONE ADJACENT THE INPUT END OF SAID COLUMN AND A COOKING ZONE SPACED FROM THE INPUT END OF SAID COLUMN BY SAID HEATING ZONE, REMOVING LIQUID FROM THE END OF SAID COOKING ZONE REMOTE FROM SAID HEATING AND COOKING ZONES AND INTRODUCING A HEATED CHEMICAL AGENT COMPRISING A SOLUTION OF SODIUM HYDROXIDE, SODIUM CARABONATE, AND SODIUM SULFIDE AND REINTRODUCING AT LEAST A PORTION OF SAID REMOVED LIQUID AT A LOCATION WITHIN THE COLUMN BETWEEN SAID HEATING AND COOKING ZONES AND REMOVING LIQUID FROM THE INPUT END OF SAID COLUMN THEREBY CAUSING LIQUID FLOW THROUGHOUT SAID HEATING ZONE IN A DIRECTION TOWARD THE INPUT END OF SAID COLUMN AND THROUGHOUT SAID COOKING ZONE IN A DIRECTION AWAY FROM THE INPUT END OF SAID COLUMN.
 4. APPARATUS FOR THE CONTINUOUS COOKING OF WOOD CHIPS COMPRISING: WOOD CHIP LIFTING MEANS MOUNTED ADJACENT THE ENTRANCE END OF SAID VESSEL TO EXACTLY LIFTING FORCE DIRECTLY ONLY ON WOOD CHIPS LOCATED AT THE ENTRANCE END OF SAID VESSEL PROGRESSIVELY TO COMPACT SAID WOOD CHIPS AND TO ADVANCE THE MASS OF SAID WOOD CHIPS PROGRESSING UPWARDLY THROUGHOUT SAID ZONES WHILE PERMITTING FREE MOVEMENT OF LIQUID THROUGH SAID MASS, WOOD CHIP DISCHARGE MEANS ADJACENT THE UPPER END OF SAID VESSEL EFFECTIVE TO MAINTAIN A PREDETERMINED LEVEL OF THE TOP OF SAID MASS OF WOOD CHIPS BELOW THE TOP OF SAID VESSEL, LOWER STRAINER MEANS ADJACENT THE BOTTOM OF SAID VESSEL UPPER STRAINER MEANS ADJACENT THE TOP OF SAID VESSEL FOR REMOVAL OF LIQUID FROM SAID VESSEL INLET MEANS FOR INTRODUCING A HEATED CHEMICAL TREATING AGENT INTO SAID VESSEL, AT A LOCATION SPACED BETWEEN SAID UPPER AND LOWER STRAINER MEANS, MEANS FOR RECIRCULATING LIQUID FROM SAID UPPER STRAINER MEANS INTO SAID VESSEL AT SAID LOCATION WITHIN SAID VESSEL SPACED BETWEEN SAID UPPER AND LOWER STRAINER MEANS SAID INLET MEANS AND STRAINER MEANS CAUSING LIQUID FLOW IN A DOWNWARD DIRECTION FROM THE INJECTION POINT OF SAID LIQUID TO SAID LOWER STRAINER MEANS AND IN AN UPWARD DIRECTION FROM THE INJECTION POINT OF SAID LIQUID TO SAID UPPER STRAINER MEANS. 